1. Field of the Invention
The present invention relates to a process for preparing an aqueous and/or alcoholic solution of hydrogen peroxide by reaction of hydrogen and oxygen over a stationary catalyst comprising a noble metal.
2. Discussion of the Background
The synthesis of hydrogen peroxide from hydrogen and oxygen over heterogeneous catalysts has been described many times. Thus, EP-A 0 579 109 describes the direct synthesis of hydrogen peroxide in a fixed-bed reactor operated in the downflow mode. Here, the catalyst is present as a fixed bed in the reactor. A gas stream comprising hydrogen and oxygen and a liquid aqueous stream are passed through the reactor in cocurrent from the top downward.
The factor which limits the formation rate of hydrogen peroxide is, at a given catalyst activity, the amount of reaction gases present at the surface of the catalyst. One restriction imposed on this is due to the requirement that, for safety reasons, the proportion of hydrogen should not exceed the lower explosive limit of hydrogen/oxygen mixtures of 4% by volume. Furthermore, the solubility of hydrogen in the liquid reaction medium, which surrounds the catalyst particles as a thin film, is low. In order to achieve an economically acceptable product yield despite these limitations, the direct synthesis of hydrogen peroxide is carried out under a high gas pressure. However, the gas pressure drops considerably along the catalyst bed because of drag forces between solid and fluid phases, i.e. between the catalyst surface and the gas phase or the liquid phase, as the distance from the gas inlet increases. To compensate for this pressure drop, an unacceptably high power input is required for compressing the gas supply.